The transition zone in concrete is a critical area where aggregate meets cement paste, marked by a distinct porosity and weakness compared to the surrounding material. The adhesion around the aggregates is primarily due to Van Der Waals forces. The voids within this zone influence its robustness; initially, it is less durable than the surrounding bulk mortar due to larger voids. Initially, when concrete is compacted, a higher water-cement ratio near the aggregates leads to the formation of water films around them, affecting the zone's porosity. This area sees the crystallization of compounds like calcium, sulfate, hydroxyl, and aluminate into ettringite and calcium hydroxide, which tend to form larger and more porous structures near the coarse aggregates due to the elevated water-cement ratio. These structures, especially the calcium hydroxide, crystallize in oriented, plate-like layers, further contributing to the porosity.
Over time, as hydration progresses, the initially large crystals of ettringite and calcium hydroxide begin to be surrounded by a denser matrix of secondary, finely crystallized C-S-H, along with smaller crystals of the same compounds, leading to a densification of the transition zone and an increase in its strength.
The transition zone, found where the aggregate meets the cement paste, is more porous and, as a result, weaker than hydrated paste beyond the transition zone.
First, when concrete is freshly compacted, films of water surround the larger aggregate particles, which can result in a greater water-cement ratio near these aggregates compared to areas further away.
Next, within the concrete's matrix, ions such as calcium, sulfate, hydroxyl, and aluminate from the dissolution of various compounds precipitate to form ettringite and calcium hydroxide.
Near coarse aggregates, where the water-cement ratio is elevated, these compounds crystallize into larger, more porous structures than those in the denser cement paste.
The calcium hydroxide often crystallizes into oriented, plate-like layers, typically perpendicular to the aggregate surfaces, contributing to the overall porosity.
Finally, as hydration advances, the space between the large initial crystals of ettringite and calcium hydroxide begins to fill with a secondary, poorly crystallized calcium silicate hydrate, along with smaller ettringite and calcium hydroxide crystals.
This secondary filling phase contributes to a denser and stronger interfacial transition zone.
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